Migration from crowded and expensive Pacific Coast cities has propelled Boise -- the capital of Idaho -- onto the list of top 10 gainers in the Bloomberg rich versus poor gap ranking of metropolitan areas from close to the bottom.

The analysis of Census Bureau data tracks the differences in annual income between household income groups. The rich versus poor gap compared households in the top 20 percent to those in the bottom 20 percent by metropolitan area.

The gap in Boise widened by $44,400 to $170,000 between 2011 and 2016 --elevating it from No. 76 to No. 7 on the Bloomberg list. For Boise, this meant that the average income among households in the top 20 percent soared to almost $182,000 while incomes in the bottom quintile stagnated around $12,000.

Affordable housing, relative to California, and short commutes are a draw as well as “a strong economic pull” with jobs in a variety of industries, said Ethan Mansfield, research and project manager at the Boise Valley Economic Partnership. However, West Coasters are pushing Boise home prices to record levels.

California led all states in Idaho migration inflows for the last three years, according to U.S. Population Migration data from the Internal Revenue Service.

Also bounding higher in dramatic fashion: Baton Rouge, Louisiana, from No. 56 to No. 10; San Diego from No. 49 to No. 9; and Sacramento from No. 32 to No. 6.

At No. 1 is San Jose, California, the Silicon Valley city where the rich versus poor gap widened by $73,600 to $339,000. At No. 100, with the smallest change among 100 largest metro areas, is the border city of El Paso, Texas, where the gap widened by $2,600 to $131,200.

Nationally, the rich versus poor gap expanded by $31,000 to just over $197,000. Last year’s measure, using data from 2010 to 2015, showed an increase of $29,500 to $189,600.

The Bloomberg ranking also shows the change in the gap between the super rich to middle class which widened in 98 of 100 metropolitan areas, led by Bridgeport, Connecticut, which overlaps entirely with Fairfield County. The gap narrowed in Ogden, Utah and Colorado Springs, Colorado. The super rich to middle class gap is defined by those in the top five percent of income vs households in the middle 20 percent.

A third take of data shows the middle class income span -- defined as the gap between those within 30 and 80 percent of an areas income. The middle class span grew the most in San Francisco where it rose to $140,800 in 2016 from $108,300 five years earlier.

The metropolitan areas where the socioeconomic gaps have diverged the most are located along the Atlantic and Pacific seaboards with Charleston, SC among the leaders as more wealthy people move to the southern coastal city.

Migration from crowded and expensive Pacific Coast cities has propelled Boise -- the capital of Idaho -- onto the list of top 10 gainers in the Bloomberg rich versus poor gap ranking of metropolitan areas from close to the bottom.The analysis of Census Bureau data tracks...